Various types of coupling or fitting assemblies are used to connect tubing together in fluid systems, such as aircraft hydraulic and fuel systems. A commonly used coupling assembly incorporates a male fitting having a conical nose which engages a female fitting having a flared or machined conical surface. A compression nut is used to produce an axial force on the male and female fittings so that the surfaces of the fittings are placed under load in metal-to-metal contact with each other.
The axial load initially applied to the fitting surfaces, i.e., the preload, produces a seal which prevents the escape of fluids which are contained under pressure within the coupling and the associated system. As long as the loading force (the preload) on the male and female fitting surfaces provides a unit loading which is greater than the pressure exerted on the surfaces by the fluid contained within the system, the seal is maintained, and no leakage occurs.
Typically, such coupling assemblies are subjected to large temperature fluctuations while in service. As the temperature varies, the components of the coupling (i.e., the male fitting, the female fitting, the compression nut and, where used, an internal lockwire holding the compression nut on the female fitting) expand and contract to various degrees, depending upon the thermal coefficients of expansion of the materials from which they are fabricated. Expansion and contraction results in stress relaxation in the components which, over time, reduces the preload on the male and female sealing surfaces. The loss of preload is more rapid when, as sometimes occurs, the components of the coupling are made of different materials with different coefficients of thermal expansion. The loss of preload continues with time, and at a rate depending on the severity of the temperature cycles, until the preload is reduced to a point at which the fluid pressure overcomes the sealing force, and leakage occurs. Such leakage can be catastrophic if it occurs in systems which contain volatile fluids or in hydraulic systems which rely on the pressure of incompressible fluids.
When there are imperfections, such as scratches or dents in the male or female sealing surfaces, the seal between the surfaces may be compromised, even when the design preload is applied or when only a small portion of the preload is lost due to thermal cycling.
In order to improve the integrity of the seal between the male and female sealing surfaces, seals which are fabricated of soft metals (such as copper, nickel or aluminum) have been interposed between the male and female coupling surfaces. The soft metal seals incorporate a conical portion, which is at the same angle as the male and female sealing surfaces. Once the soft metal seal is in place between the male and female surfaces, an axial force is applied to the fittings by the compression nut, thereby drawing the surfaces toward each other and crushing the soft metal seal therebetween. The metal flows into any surface imperfections in the male and female surfaces, thereby improving the seal integrity.
Although the use of a soft metal seal alleviates problems associated with seal surface imperfections, it does not alleviate the problem associated with loss of preload between the surfaces due to thermal cycling. This is because the soft metal seal does not independently exert any force on the sealing surface once the preload is lost.
It is therefore desirable to provide a seal having a shape and fabricated from a material so that the combination of the shape and the properties of the material enable the seal to exert a sealing force on the male and female surfaces of the coupling assembly in which it is incorporated, even when the assembly preload is lost or is substantially reduced.